Variable impedance transformer



oct. s, 1945. D. c. ESPLEY' '2,408,145

VARIABLE IMPEDANCE TRANSFORMER Filed Nov. 10, 1942 Fig. 2. A7

Fig.v 3.

Fig. 5.

Patented Oct. 8, 1946 VARIABLE IMPEDANCE TRANSFORMER Dennis Clark Espley, North Wembley, England, assignor to The General Electric Company Limited, London, England Application November 10, 1942, Serial No. 465,143 In Great Britain November 11, 1941 13 Claims. l

This invention relates to impedance transformers of the type in which part at least of the impedance transformation is effected in a transmission line. The proposition upon which the action of such impedance transformers depends is this. Let a load impedance Z be connected to given input terminals through a transmission line or a set of transmission lines connected in series; then by a suitable choice of the length and characteristic impedances of the line or lines, the input impedance looking into that end of the transmission line which is connected to the given input terminals can be given any assigned value Zn. When the suitable choice is made, the line or lines are said to transform Z to Zn.

The object of this invention is to produce a I simple and convenient variable transformer of this type.

In accordance with one form of the invention, a variable-impedance transformer of the type in which part at least of the impedance transformation is effected in a transmission line comprises a transmission line having a first pair of terminals adapted to have coupled thereto an impedance of a first value and having a second pair of terminals at which it is desired that the first value of impedance be transformed by the line to a second value of impedance. The transformer includes modifying means readily slidable along the transmission line in the operative condition thereof to modify the characteristic impedance of at least two substantial parts of the line which parts have positions along the line selectable by movement of the modifying means.

Also in accordance with the invention, a highfrequency impedance-matching device adapted for connection between and for matching at a given frequency a, plurality of impedances having any values within a substantial range of magni-l tude and phase comprises a plurality of substantially parallel .conductors adapted to be connected between the impedances and having a predetermined characteristic impedance within the aforesaid range between a predetermined pair of the aforesaid conductors. The device includes a plurality of means between the conductors and individually adjustable axially therealong, each of the adjustable means 'being of such shape and material as to alter the aforesaid characteristic impedance for matching the resistive components and canceling the reactance components of the aforesaid plurality of impedances over the aforesaid range of values.

In the accompanying drawing:

Fig. 1 is an explanatory diagram of a trasmission line,

Fig. 2 shows diagrammatically one embodiment of the invention, and

Figs. 3, 4, 5 and 6 show diagrammatically parts of alternative embodiments, given by Way of example.

The principle underlying the invention will now be described with reference to Fig. 1, which shows a transmission line which, in the absence of the means for modifying its characteristic impedance, has a characteristic impedance Zu; the wave-length of the oscillations translated by the line, where the characteristic impedance is Zo, is l. At the left hand end the line is shownin conventional manner as being terminated by the complex impedance R14-irl. The portions of the line between the sections B, C and D, E thereof are each modified so that they have characteristic impedance Z1; the lengths of these sections are each A74, where i is .ther wave-length of the oscillations in these sections. Sections C and D are separated by a space of length l1 over which the characteristic impedance of the line is Z0. In order that the line may be matched to an impedance Z0 at the right hand end, the impedance ZE looking into the section E towards the left must be Z0. The problem is to determine the conditions under which this condition can be fulfilled by adjusting suitably'the distance l1 between the two modified parts of impedance Z1 of the line and the distance of the section B from the left-hand end of the line.

The known proposition appropriate to the problem is this. Consider two sections U and V across a uniform transmission line of characteristic impedance Z, these sections being separated by a distance .'12 positive in the direction from U to'V. Then if ZU, Zv are the impedances looking into these sections in the direction in which :c is positive ZV'i'jZ tall x ZZ(Z+jzv' an (1) where =21r/i\ and i is the wave-length of the oscillations along the line,

It is a consequence of Equation 1 that some section A can be found, distant a: from the lefthand end of the line shown in Fig. 1, at which the impedance looking to the left is wholly real, say R. It is sufficient therefore to discuss how, if at al1, the distance l between the sections A, B and the distance l1 may be adjusted so as to transform the real impedance R at A into the impedance Zo at E. Applying Equation 1 to the 3 region between sections A and B, the impedance at B looking to the left is given by R-i-ZU tan ZnijR tall h Applying Equation l to the region between sections B and C, the impedance at C looking to the left is given by Applying Equation 1 to the region between sections C and D, the impedance at D looking to the left is given by Zeri-.izo tall li ZDIZWOMZC tan et (4) Finally for ZE Z Z,.- ZD 5) Putting ZE==Zu, eliminating ZB, Zo and ZD from (2), (3), (4), (5), and writing lc=Zn2/Z12 (6) Equating real parts and imaginary parts,

(ZO- kilt) (R- k2Z0) In order that the desired transformer ratio may be possible, l and Z1 must be real, and the righthand sides of both (7) and (8) must be positive; that is to say (Zu-ICZR) and (R-lc2Zo) must be of the same sign, which implies very complicated and would be of little or no value A in practice. Accordingly only a few other ways of modifying the line will be mentioned specifically. If the modified sections of the line were again two in number and each of the same characteristic impedance, but each of length x78,

the range of values of R that could be transformed to Zn would be k2 to 1. Again if the modified sections were each of length i/4, but had diierent characteristic impedances Z1 and Z2, then the said range would be k12lc22 to l, where Again `the lengths of the two modified sections need not be equal. Again the line in the absence of the modifying means need not be uniform; it may be modified permanently at certain places, so as tu have a different characteristic impedance at these places; these places may lie between two movable modified sections or outside both of them. In particular a modified section having a length W/2, where A" is the wave-length in the section and n is an integer, may obviously be introduced anywhere along the line without substantially modifying the foregoing theory. Indeed the characteristic impedance of the modified line might vary continuously along the line; but no advantage is known in this suggestion Lastly there might be more than two modified sections. By this means the range of the ratio of transformation corresponding to a given 7c can be increased, but the difficulty of adjustment to give a desired ratio increases also. For when there are only two modied sections, two variables l and Z1 have to be adjusted; the adjustment has to be made by double trial and error, ke the adjustment of a bridge to balance for both A. C. and D. C. This is feasible, but the adjustment would be very laborious with (say) three modled sections and three variables.

Those skilled in the art will realise how the characteristic impedance of the line may be modied at adjustable places. One method is to pr=2- vide blocks, slidable along the line, and of a material having a permittivity whose ratio to the perrnittivity of the medium intervening between the conductors on the unmodified part of the line is K, and substantially different from 1. Then, if the dielectric fills all the space occupied by the oscillating eld 7c=K. Suitable materials are known for which K=2.5 relative to air; hence if there are two blocks, each 7\/4 long, the ratio of transformation can be varied over a range (2.5) 4 to l, i. e. about 40 to l.

In the embodiment shown in Fig. 2 the line is concentric and the modifying means are dielectric blocks. I and 2 are inner and outer members of a concentric line, which has the characteristic impedance Z0 in the absence of the blocks. The rectangle marked Z indicates diagrammatically the load impedance connected to one end of the line. 5 and E are similar blocks, each made of the material known commercially as Distrene, which has a dielectric constant K relative to air of about 2.5. The length of each is A74 where A is the 'wave-length of the oscillations in the block; if

A is the wave-length in the absence of the block, the length is A/iKl Each block is slidable along the line by means of threaded pins 1 projecting through slits in the member 2 at opposite ends of a diameter. When they are adjusted, the blocks are then clamped in place by means of nuts on the threaded pins 1.

If the ratio of the internal diameter of the member 2 to the diameter of the member l is 3.5 to 1, Z0 will be 75 ohms and any load resistance between 12 and 470 ohms can be transformed to '75 ohms when the blocks 5 and 8 have the length and composition last described.

Other methods of modifying the characteristic impedance of the line at adjustable places are shown in Figs. 3 to 6. Fig. 3 shows a metal sleeve 9 slidable along the inside of the outer conductor of a concentric line and Fig. 4 shows a metal sleeve I0 slidable along the outside of the inner conductor of a similar line, IDA denoting a shifting and locking member of insulating material. Since such sleeves will decrease the ratio of the inner diameter of the outer conductor to the outer diameter of the inner conductor, the characteristic impedance will be decreased. Fig. 5 shows a parallel line lA, 2A having a metal sleeve I l slidable on one conductor, and Fig. 6 shows a similar line having a dielectric block l2 slidable on both conductors.

I claim:

l. A variable impedance transformer of the type in which part at least of the impedance transformation is effected in a transmission line comprising, a transmission line having a first pair of terminals adapted to have coupled thereto an impedance of a first yvalue and having a second pair of terminals at which it is desired that said first value of impedance be transformed by said line to a second value of impedance, and modify ingmeansreadily slidable along said transmission line in the operative condition thereof to modify the characteristic impedance of at least two substantial parts of said line which parts have positions along said line selectable by movement of said modifying means.

2. A variable impedance transformer of the type in which part at least of the impedance transformation is effected in a transmission line comprising, a transmission line having 'a first pair of terminals adapted to have coupled thereto an impedance of a first value and having a Second pair of terminals at which it is desired that said first value of impedance be transformed by said line to a second value of impedance, and

blocks separately slidable along said line and of a permittivity different from that of the medium intervening between the conductors along the unmodified part of the line, said blocks constituting modifying means whereby the characteristic impedance of at least two substantial parts of said f line may be modified.

3. A variable impedance transformer of the type in which part at least of the impedance transformation is effected in a transmission line comprising, atransmission line having a first pair 1 of terminals adapted to have coupled thereto an impedance of a first value and having a second pair of terminals at which it is desired that said first value of impedance be transformed by said line to a second value of impedance, and metal sleeves slidable along at least one of the conductors constituting said transmission line, said sleevesconstituting modifying means whereby the characteristic impedance of at least two substantial parts of said line may be modified.

4. A variable impedance transformer of the type in which part at least of the impedance transformation is effected in a transmission line comprising, a transmission line having a first pair of terminals adapted to have coupled therei to an impedance of a first value and having a second pair of terminals at which it is desired that said first Value of impedance be transformed by said line to a second value of impedance, and

two blocks separately slidable along said line and of a permittivity different from that of the medium intervening between the conductors along the unmodified part of the line, the length of each of said blocks being one-quarter of the wavelength, in a modified part of the line, of the oscillations in connection with which the transformer is adapted to be used.

5. A variable impedance transformer of the type in which part at least of the impedance transformation is effected in a transmission line comprising, a transmission line having a first pair of terminals adapted to have coupled thereto an impedance of a first value and having a second pair of terminals at which it is desired that said first value of impedance be transformed v:

by said line to a second value of impedance, and two metal sleeves slidable along at least one of the conductors constituting said line, the length of each of said sleeves being one-quarter or the wave-length, in a modified part of the line, of

the oscillations in connection with which the transformer is adapted to be used.

6. A variable impedance transformer of the type in which part at least of the impedance transformation is effected in a transmission line fl d comprising, a concentric transmission line having a first pair of terminals adapted to have coupled thereto an impedance of a first value and having a second pair of terminals at which it is desired that; said first value of impedance be transformed by said line to a second value of impedance, two modifying elements disposed within the outer conductor of said line for modifying the characteristic impedance of two substantial parts of said line, and means extending out of said outer conductor for independently varying the positions of said elements along said line.

7. A high-frequency impedance-matching device adapted for connection between and for matching at a given frequency a plurality of impedances having any values within a substantial range of magnitude and phase comprising, a plurality of substantially parallel conductors adapted to be connected between said impedances and having a predetermined characteristic impedance within said range between a predetermined pair of said conductors, and a plurality of means between said conductors and individually adjustable axially therealong, each of said adjustable means being of such shape and materials as to alter said characteristic impedance for matching the resistive components and cancelling the reactance components of said plurality of impedances over said range of Values.

8. A high-frequency impedance-matching device adapted for connection between and for matching at a given frequency a plurality of impedances having any values within a substantial range of magnitude and phase comprising, a plurality of substantially parallel conductors adapted to Fbe connected between said impedances and having a predetermined characteristic impedance within said range between a predetermined pair of said conductors, and a plurality of means between said conductors and individually adjustable axially therealong, each of said adjustable means being of such shape and material as to modify said characteristic impedance over at least one limited distance along said conductors for matching the resistive components and cancelling the reactance components of said plurality of impedances over said range of values.

9. A high-frequency impedance-matching device adapted for connection between and for matching at a given frequency a plurality of impedances having any values within a substantial range of magnitude and phase comprising, a hollow outer conductor and an inner conductor substantially coaxial therewith toprovide a transmission line having a predetermined characteristic impedance within said range and adapted to be connected between said impedances, and a plurality of means between said conductors and individually adjustable axially therealong, each of said adjustable means being of such shape and material as to alter said characteristic impedance for matching the resistive components and cancelling the reactance components of said plurality of impedances over said range of values.

l0. A high-frequency impedance-matching device adapted for connection between and for matching at a given frequency a plurality of impedances having any values within a substantial range of magnitude and phase comprising, a hollow outer conductor and an inner conductor substantially coaxial therewith to provide a transmission line having a predetermined characteristic impedance within said range and adapted to be connected between said impedances, and a plurality of independently axially adjustable means between said conductors surrounding and substan' tially coaxial with said inner conductor, each of said adjustable means being of such shape and material as to alter said characteristic impedance for matching the resistive components and cancelling the reactance components of said plurality of impedances over said range of values.

l1. A high-frequency impedance-matching device adapted for connection between and for matching at a given frequency a plurality of impedances having any values within a substantial range of magnitude and phase comprising, a plurality of substantially parallel conductors adapted to be connected between said impedances and having a predetermined characteristic impedance within said range between a predetermined pair of said conductors, and a plurality of conductive annular means between said conductors and individually adjustable axially therealong, each cf said adjustable means being effective to alter said characteristic impedance over a limited distance along said conductors for matching the resistive components and cancelling the reactance components of said plurality of impedances over said range of values.

12. A high-frequency impedance-matching device adapted for connection between and for matching at a given frequency a plurality of iinpedances having any values within a substantial range of magnitude and phase comprising, a hollow outer conductor and a cylindrical inner conductor substantially coaxial therewith to provide a transmission line having a predetermined charactertistic impedance within said range and adapted to be connected between said impedances, and a plurality of cylindrical conductive means between said conductors and individually adjustable axially therealong, each of said adjustable means being of such shape and material as to alter said characteristic impedance for matching the resistive components and cancelling the reactance components of said plurality of impedances over said range of values.

13. A high-frequency impedance-matching device adapted for connection between and for matching at a given frequency a plurality ol im pedances having any values within a substantial range of magnitude and phase comprising, a plurality of substantially parallel conductors adapted to be connected between said impedances and having a predetermined characteristic impedance within said range between a predetermined pair of said conductors, and a plurality 0I dielectric elements between said conductors and individually adjustable axially therealong, each of said elements being of such shape and having such a dielectric constant as to alter said characteristic impedance for matching the resistive components and cancelling the reactance components of said plurality of impedances over said range of values.

DENNIS CLARK ESPLEY. 

